1. Field of the Invention
The invention relates to a linear motor.
2. Description of the Prior Art
Linear motors comprise a stator and a runner which is displaceable in the direction of the longitudinal axis of the stator. In this the runner can in principle be designed as an inner runner or as an outer runner. A known system of linear motors comprises one or more electronics units and linear motors which are connected to the latter. The electronics units represent, on the one hand, a connection to a higher level control system and are, on the other hand, responsible for the control or the regulation respectively of the linear motors which are connected to them.
In order to be able to take over the control or the regulation respectively of the linear motor the electronics unit must continually receive information on the current position of the runner. For this purpose for example two Hall sensors are mutually displacedly arranged in the stator in a typical exemplary embodiment of a linear motor and indeed in such a manner that they convert the magnetic field produced by the magnets of the runner into an electric sine or cosine output signal respectively as a result of their displaced arrangement. Furthermore, in this exemplary embodiment an amplifier circuit is also provided in the stator which amplifies the electric output signals of the Hall sensors so that it is also possible to transmit the comparatively weak output signals of the Hall sensors over a longer connection cable to the electronics unit. Otherwise only passive components are arranged in this exemplary embodiment, such as for example temperature sensors and naturally the coils of the linear motor.
It is immediately evident that the linear motors and the electronics units must be exchangeable among one another as desired; thus a linear motor of the same type must be able to function without problem with every electronics unit which is provided for its operation. This must already be the case since in the event of a defect in a linear motor or in an electronics unit the respective other non-defective component can be used unchanged.
This means that the linear motors and their individual components must be manufactured with manufacturing tolerances which are as low as possible--that is, with a high precision of reproduction. Then namely the control and regulation parameters can remain unchanged in the electronics unit and need not be determined anew. In practice however manufacturing tolerances are always to a certain extent present in the manufacture and cannot be completely avoided. This will be explained in the following.
The ideal state is first illustrated in FIG. 1. One recognizes there the output signals 1 and 2--the output voltages U--of two Hall sensors which are thus displacedly arranged with respect to one another and the sine- or cosine-shaped output signal respectively which is produced by the magnets of the runner, plotted as a function of the time t.
Hall sensors can have a certain offset voltage U.sub.offset, which is superimposed at the output of the Hall sensor as a d. c. voltage component on the sine or cosine signal respectively which is produced by the magnets of the runner. This can be recognized in FIG. 2, where the signal 2 is displaced by the offset voltage U.sub.offset in the direction of the ordinate with respect to signal 1 (ideal signal without offset voltage).
Furthermore, Hall sensors can also have a phase shift offset .phi..sub.offset, as is indicated in FIG. 3. There the signal 2 is displaced in the direction of the abscissa relative to the signal 1 by the phase shift offset .phi..sub.offset.
Furthermore, Hall sensors can also have a different amplification factor. Thus with the same excitation of the runner by the magnets, one Hall sensor has an output signal--an output voltage U--which is less in amplitude than the amplitude of the output signal of the other Hall sensor. In FIG. 4 one recognizes such an example of two Hall sensors with different amplification factor: The amplitude of the signal 1 of the one Hall sensor is greater than the amplitude of the signal 2 of the other Hall sensor.
A further imprecision can result from the magnets of the runner. One recognizes in FIG. 5 that the distances d1 and d2, which in each case designate the spatial distance of two like magnetic poles (here: of two north poles), can vary slightly; the magnets have--when viewed in the direction of the movement of the runner--different lengths. In the example shown in FIG. 5 the maximum amplitude of the output signal of the Hall sensors is already achieved after a time t which corresponds to a phase angle of 350.degree. when the speed is uniform; the distance d2 is thus less than the distance d1. If the like magnetic poles (here the north poles) were always equally spaced, the maximum amplitude would not arise again until a time t which corresponds to a phase angle of 360.degree..
Imprecisions can however also result from the mechanical arrangement of the Hall sensors. The Hall sensors HS 1 and HS2 are (see FIG. 6) arranged in a holder 3 in depression 31 and 32 ideally at a distance a from one another which corresponds to a phase angle of 90.degree. when the distance of the magnets of the runner remains the same. As a result of this a sinusoidal output signal is produced by the one Hall sensor HS1 when passing the magnets of the runner, whereas a cosinusoidal output signal is produced by the Hall sensor HS2. This corresponds to the ideal case (see also FIG. 1). If the distance a does not exactly correspond to a phase angle of 90.degree. then a phase shift offset, which was already explained with reference to FIG. 3, can arise.
A further cause for a phase shift offset can however also come from the fact that the actual Hall sensors HS1 and HS2 are not precisely centrally arranged inside the sensor housing. Such a case is drawn in FIG. 7, with the arrow crosses in each case indicating the direction and the amount of the displacement. The right sensor HS2 is thus arranged inside the depression 32 (not illustrated in FIG. 7, see FIG. 6) with a displacement to the right and to the front.
As already mentioned the Hall sensors HS1 and HS2 can have different amplification factors. Whereas the cause of this can be component-immanent--that is, the Hall sensors HS1 and HS2 have a different amplification factor in themselves at an ideal arrangement within the depressions 31 and 32--the different amplification factor can also result from an inclined installation position--in FIG. 8 e.g. of the Hall sensor HS2 within the depression 32.
On the whole it can thus be concluded that quality differences--if often only of slight extent--are always present and therefore reductions must be made or, respectively, compromises must be made in the design of the regulation and the reliable performance features of the linear motors.
It is furthermore disadvantageous in known linear motors that the latter are regularly labelled with stickers from which it can be determined of which type the linear motor is or, respectively, which variant within a type the linear motor is. If the writing on the label becomes illegible, then certain parameters of the linear motor can often be determined either only with difficulty or not at all. The serial number is in principle lost in such a case since a separate serial number is issued for each motor. This is disadvantageous precisely in the case of a necessary recall of linear motors since under certain conditions the affected linear motors can no longer be identified.
It is admittedly conceivable in principle to stamp the serial number into the housing (like the chassis number in automobiles), but this is however not practicable from the commercial side (cost and complexity).
As far as the respective variant of the linear motor within a type of linear motor is concerned, the different variants often have an absolutely identical appearance from the outside, but have a fundamentally different electrical "inner life". If the user then selects the false variant in the electronics unit, this can lead to faulty functioning. Previous attempts to solve this problem go in the direction of attempting to uniquely identify the motor type and the motor variant as a result of the signals which are measurable by the electronics unit. This is however not always possible for technical reasons.